Conventional methods of fabricating ohmic contacts to bi-polar semiconductor devices require an exclusive application of one group of metallization on n-type contact areas of a device, and also require another group of metallization on p-type contact areas of the device. This characteristic is governed largely by the work function (WF) of the metal, relative to that of the semiconductor.
Theoretically, and in general, for n-type semiconductors, ohmic contact is accomplished when the WF of the metal is lower than the WF of the semiconductor. A Schottky barrier, or rectifying contact, is formed when the WF of the metal is greater than the WF of the n-type semiconductor. For p-type semiconductor, ohmic contact is achieved when the metal WF is greater than the WF of the semiconductor while a Schottky contact is formed, conversely.
The solid state physics of thermionic emission charge transport mechanism that supports the above conditions is not applicable in cases where the semiconductor, n-type or p-type, is degenerately doped, that is in cases when the impurity concentration is so high that the Fermi energy level is above the conduction band minimum or below the valence band maximum, respectively. Under such condition, the depletion width narrows, i.e., becomes electronically transparent, to allow charge tunneling that require minimum thermal energy. Under this condition, the exclusivity of the metal to form ohmic contact is removed and a broader range of metals can promote ohmic contact to either degenerately doped n-type or p-type semiconductors.
A bi-polar semiconductor device, such as a p-n junction device, is a building block for complex integrated circuits (IC) used in broad range of electronic applications. The bipolar semiconductor requires ohmic contacts to be formed on the n-type and p-type contact surfaces. Because mid to low impurity concentrations are needed to form a high performing bipolar device, it would require the exclusive use of a group of metals having lower WF, for the n-type surface, and the exclusive use of another group of metals with higher WF, for the p-type surface.
Conventionally, this requires multiple fabrication process steps with several masks to finally obtain the desired ohmic contacts. These steps include high temperature processing, photolithography, chemical etching, and others. The above batch of steps is repeated in order to finally obtain the ohmic contacts. The current processes are time consuming and complex. They are also characterized by high production costs, and the final device yield on the wafer is potentially reduced due to the large number of process steps.
In some cases, one of the layers is deliberately degenerately dope such that the metal ohmic contact exclusivity is removed, making it possible for broader range of metals to be ohmic on both n-type and p-type layers. One process of forming a degenerately doped layer in the contact area is by high energy ion implantation. This entails surface preparation prior to ion implantation, usually performed at temperatures over 1000 degrees Celsius. This process not only increases cost, but also induces damage to the lattice structure of the semiconductor. The induced damage can only be partially reversed after annealing. Post anneal ion implantation also required that the implant be activated at a temperature higher than 1200 degrees Celsius. It is also known that not all implants are fully activated even at 1200 degrees Celsius.
Thus, an alternative approach to solving this problem may be beneficial.